A vehicle height adjusting device for a vehicle hydraulic shock absorber is publicly known for example from Japanese patent laid-open publication Sho-58-56985. As shown in FIG. 9, such hydraulic shock absorber for vehicles, to which the self pumping type vehicle height adjusting device is attached, is a rear cushion which comprises a cylinder 72 having at one end thereof a mounting member, a piston 73 slidably fitted in the cylinder 72, a piston rod 75 protruding from the piston and having at the free end thereof a mounting member 74, a fluid tank 76 having a reservoir D connected to a hydraulic chamber A between the piston and the cylinder, and a suspension spring 77 interposed between each of the spring bearings of the cylinder and the piston rod. The automatic vehicle height adjusting device comprises, as a hydraulic jack unit, a plunger case 78 firmly connected to the mounting member of the rear cushion and a plunger 79 slidably fitted on the outer periphery of the plunger case 78, and hydraulic jack chamber B is formed between the spring bearing at the inner end of the plunger and the plunger case. A check valve 80, which is adapted to be opened at the compression stroke of the shock absorber and closed at the extension stroke, is provided at the end of the piston rod within the cylinder, and the check valve 80 communicates with the hydraulic jack chamber B through a passage 81 provided concentrically in the piston rod 75. The cylinder 72 is provided at the bottom thereof with a further check valve 82, and the hydraulic chamber A within the cylinder is connected through the check valve 82 to a reservoir C between the cylinder 72 and the outer tube 83 and/or to a reservoir D of the fluid tank 76.
During the running of the motorcycle, a portion of the hydraulic fluid corresponding to the volume taken by the piston rod 75 which enters into the cylinder at the compression strokes of the shock absorber passes through the orifice of a compression side damping valve 82 at the bottom of the cylinder 72 and forces the disc valve adjacent to the lower surface of the damping valve 82 to be deflected, thereby allowing said portion of the hydraulic fluid to flow therethrough to the reservoir D between the cylinder 72 and the outer tube 83 and/or to the fluid tank 76 at the outside, while producing the damping force at the compression side. At the same time, another portion of the hydraulic fluid passes through a valve arranged on the upper surface of the piston 73 into the upper chamber above the piston, while producing the damping force at the compression side. Furthermore, at the compression stroke, the check valve 80 within the valve housing at the end of the piston rod 75 is kept open, so that the hydraulic fluid within the hydraulic chamber A is allowed to flow through the passage 81 into the hydraulic jack chamber B. At the extension stroke, the check valve 80 is closed. Consequently the self-pumping action is produced, so that the hydraulic fluid is introduced into the hydraulic jack chamber B at every compression strokes to push the plunger 79 upwardly to thereby increase the vehicle height.
However, this self-pumping type vehicle height adjusting device enables the vehicle height to be kept constant, but it could not satisfy a demand that when the motorcycle is running on a waste land, the vehicle height is increased to enhance the capability of running, and when the motorcycle is running on the streets, the vehicle height is decreased to facilitate the rider's leg to reach the earth.
Furthermore, the amount of the fluid corresponding to the volume taken by the piston rod which enters into the cylinder is allowed to flow from the hydraulic chamber A within the cylinder partly to the passage leading to the hydraulic jack chamber B and partly to the passage leading to the reservoir D of the fluid tank 76 through the demping valve 82. In this way, since the two passages from the compression pump chamber A are arranged in parallel, the amount of the hydraulic fluid flowing into the jack chamber B is made less by the portion returned to the reservoir, which takes longer time for the up of vehicle height. Besides, such parallel arrangement of the jack chamber and the reservoir makes difficult the setting of the damping force in the compression side damping valve mechanism, for example when the flow rate to the hydraulic jack chamber B is made greater for more quick rate in the increase in vehicle height, it is necessary to make greater the throttle of the compression side damping valve 82 or a pressure at which the valve is caused to open, but which will result in much higher damping force at the compression stroke and therefore in a harder ride. On the contrary, more soft setting of the compression side damping force will result in less amount of fluid delivered to the hydraulic jack chamber B, which requires longer time for the increase in vehicle height, and accordingly the meaning of the vehicle height adjusting device attached to the motorcycle will be lost.
Moreover, since the amount of the fluid corresponding to the volume taken by the rod which enters into the cylinder is divided into two ones as stated above, the fluid passing through the compression side damping valve is a portion thereof, and therefore the amount of the fluid which contributes to the generation of the damping force is less and the damping force at each of the compression strokes during raising of the vehicle height is not sufficient so that bottoming is easily caused.